Relatively inexpensive silica sand grains bound together with a suitable binder is used extensively as a mold and core material for receiving molten metal in the casting of metal parts. Olivine sand is much more expensive than silica sand but provides cast metal parts of higher quality, particularly having a more defect-free surface finish, requiring less manpower after casting to provide a consumer-acceptable surface finish. Olivine sand, therefore, has been used extensively as a mold and core surface in casting non-ferrous parts in particular and has replaced silica sand in many of the non-ferrous foundries in the United States
Spherical or ovoid grain, carbon or coke particles also have been used as foundry sands where silica sands and olivine sands do not have the physical properties entirely satisfactory for casting metals such as aluminum, copper, bronze, brass, iron and other metals and alloys. Such a carbon sand presently is sold by American Colloid Company of Arlington Heights, Ill. under the trademark CAST-RITE.RTM. and has been demonstrated to be superior to silica sand and olivine sand for foundry use.
The carbon sand used to date in the foundry industry, however, is relatively expensive to thermally stabilize so that the carbon foundry sand does not shrink or expand excessively when heated to the temperature of the molten metal that the sand is in contact with during casting. Expansion/contraction of a sand mold or core when heated to the elevated temperatures of molten metals may result in cracks in cores and molds and veining and metal penetration defects in the surfaces of the cast metal parts. Thus, the thermal stability of carbon sand is highly beneficial and is recognized as being superior to silica and olivine sands.
An inexpensive source for carbon particles useful as a carbon foundry sand is fluid coke that is a by-product of the petroleum refining industry. This petroleum refinery coke, or "raw fluid coke", is formed in a fluidized bed petroleum refining process and contains about 5% by weight petroleum hydrocarbons that volatilize into gases at the temperature of many molten metals, such as aluminum, copper, brass, bronze, and iron. During the casting of molten metals against raw fluid coke, evolving gases can bubble into the liquid metal and remain as cavities in the solidified casting, causing the casting to be scrapped.
To perform as a superior foundry sand, therefore, carbon sand should receive sufficient heat treatment to remove most of the volatile matter and to render it more thermally stable than both silica sand and olivine sand. Prior art carbon sands, therefore, have been devolatilized and pre-shrunk using an expensive, very high temperature heat treatment or calcining process at a temperature of about 2000.degree. F. to 2800.degree. F. A general description of the source and process of preparing and heat-treating the spherical or ovoid grain carbon sand is described in U.S. Pat. Nos. 2,830,342 and 2,830,913, which patents are hereby incorporated by reference.
In accordance with the present invention, it has been found that a spherical or ovoid raw fluid carbon or coke, e.g. petroleum-derived, as described in U.S. Pat. Nos. 2,830,342 and 2,830,913, having a suitable particle size for a foundry molding sand, can be roasted at a temperature of about 1000.degree. F. to about 1500.degree. F., particularly about 1200.degree. F. to about 1400.degree. F, e.g. 1300.degree. F., to provide an unexpectedly superior spherical or ovoid carbon foundry sand that produces unexpectedly superior cast or molded metal parts. The roasted carbon foundry sand of the present invention is unexpectedly superior to carbon foundry sands that have been calcined at temperatures of 2000.degree. F. and above, particularily for casting aluminum, brass and bronze.